EUV light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, indium, antimony, tellurium, aluminum, etc., with an emission line in the EUV spectrum. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
Heretofore, various systems in which a line-emitting element is presented for irradiation/electric discharge have been disclosed. Many diverse forms and states have been attempted, to include, presenting the element in pure form, e.g., pure metal, presenting the element as a compound, e.g., a salt, or in a solution, e.g., dissolved in a solvent such as water. Moreover, systems have been disclosed in which the line-emitting substance is presented as a liquid, including relatively volatile liquids, a gas, a vapor and/or a solid, and can be in the form of a droplet, stream, moving tape, aerosol, particles in a liquid stream, gas jet, etc.
One factor that is often considered when designing a high volume EUV light source is the generation and mitigation of debris inside the light source which may adversely affect the light source. For example, the debris may damage EUV light source optics, e.g., the laser input window, collector mirror and/or metrology equipment, may absorb/interfere with the transmission of EUV light within the light source, and/or may cause damage to downstream components such as the components of the illuminator/projection optics used to expose a semiconductor. These debris can include out-of-band photons, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material, and for volatile source materials can include gasses and/or vapors. Typically, these debris are emitted in all directions from the irradiation site, however, in some cases, a significant portion of the irradiated source material may be directed in the same general direction as the laser beam after irradiation. In the case of a volatile source material, the source material may continue to produce gas/vapor after passing through the irradiation site. Moreover, precautions may be necessary to prevent the laser beam exiting the irradiation site from interacting with the optics downstream of the light source, e.g., the illuminator/projection optics.
With the above in mind, Applicants disclose a source material collection unit for a laser produced plasma EUV light source and corresponding methods of use.